1. Field of the Invention
The present invention relates to a controller of a synchronous motor, and more specifically to a controller capable of detecting position of magnetic poles of a movable member of a synchronous motor (a rotor in the case of a rotary synchronous motor and a linear movable member in the case of a linear synchronous motor).
2. Description of the Related Art
In a synchronous motor, it is required to detect a position of a magnetic pole of a movable member since control is performed by detecting the position of the magnetic pole of the movable member to determine an excitation phase for applying an electric current. In order to detect the position of the magnetic pole, dedicated sensors such as an encoder, a resolver and a Hall element for detection of position of the magnetic pole have been conventionally used. However, dedicated sensors for detection of position of a magnetic pole are expensive. Moreover, it is required to perform the positioning of the sensor and the magnetic pole position of the motor, and the work of installing the dedicated sensor is also difficult, resulting in depravation of productivity of the motor.
Consequently, a detecting method for detecting position of the magnetic pole by using a sensor such as a position detector and a velocity detector for detecting a position and velocity of a rotor, without using a dedicated sensor for detection of the magnetic pole position, has been developed. This method includes the steps of applying a direct current to the given excitation phase, detecting a direction of motion of a movable part at the time of the application of the direct current, estimating the magnetic pole position of the rotor, and determining the subsequent excitation phase based on the predicted magnetic pole position, and applying the direct current thereto. This operation is repeated to detect the magnetic pole position of the rotor (see JP 2001-78487A, for example).
According to the method of detecting a magnetic pole position utilizing a sensor for detecting the position and velocity of a rotor or a movable member of a synchronous linear motor without using a dedicated sensor for detection of a magnetic pole position, a direct current is flown at a predetermined excitation phase, and the magnetic pole position is predicted on the basis of the displacing direction of the rotor or the movable member. When the direct current is applied, however, the rotor or the movable member is moved by a large amount in spite that the motion of the rotor should take place within the given range, which occasionally causes an operation abnormality.
Specifically in tandem control in which one axis, namely one object to be driven, is driven by two or more motors, if only one of the motors is provided with a sensor for detecting a position and velocity, when a magnetic pole position is detected using the sensor mounted on one of the motors, the axis (object to be driven) driven by two or more motors twists or bends on account of a disparity between the magnetic pole position of the motor and the magnetic pole positions of the other motors. As a consequence, it is sometimes impossible to detect an exact position of the magnetic pole.